The invention relates generally to battery backed-up power supply systems and more specifically to methods and systems for testing battery connectivity in such battery backed-up systems.
Reliable and quality power is essential for smooth functioning of industrial and non-industrial systems like hospitals, utilities, telecommunication systems, airlines, railways, operations in manufacturing sites and several other operations. Besides the failure of supply of power, even the voltage sags and spikes in the power supply affect the quality of power and these may have detrimental effect on the systems or equipments being supported by such a power supply.
Power management systems such as the uninterruptible power supply (UPS) systems typically allow the main utility power either directly or through converters, to supply the connected load during periods of availability of high quality generated electric power. However, when there is power loss or power is of poor quality, these systems switch to an alternate source of electric power to generate the required output for the connected loads.
Typically, the alternate source is in the form of batteries. Even in systems that utilize a motor-driven electric power generator, batteries are used to bridge the gap between the loss of utility power and the availability of the motor-driven generator. Availability of the battery back-up during power breakdown, including when the quality of power is poor, is very critical for normal functioning of any system. Typically, the electric power storage batteries include a number of individual battery cells coupled in series to generate the output voltage required for the system. Since each of the individual battery cells are required to generate the proper output voltage, the presence of an undetected failed cell may result in a system malfunction during periods of power outage when the batteries are used to supply power to the connected load. Alternatively the duration and quality of power supplied by the batteries may not be sufficient to drive the load appropriately. Hence reliability of these critical systems depends on the health and connectivity or presence of the battery bank, at all times. Additionally, the connectivity or presence of battery bank should be monitored frequently, to ensure that there is no open circuit in the path of energy storage for reasons like circuit breaker open, loose connection or open cell etc.
Generally, the techniques used for monitoring the state of batteries or for testing the battery connectivity typically involve discharging the battery. This discharging in turn affects the life of the battery. Typically, normal maintenance is carried out 2-4 times a year and cell voltages and specific gravities are measured in float charging conditions. A load test is also performed once every 1-3 years. Additionally, the connections and internal corrosion of a battery are measured by impedance and conductance measurements. In one such technique, 70-80% of the battery capacity is discharged and a voltage deviation between a fixed reference voltage and a middle-point voltage of the battery string is measured. Another technique monitors a resistance component of the battery bank by monitoring the relationship between a voltage drop across the entire battery bank, based on audio frequency injected current. In another technique, the state of charge is measured by using a separate power supply. In this technique, the measurement affects the total battery voltage and the load voltage.
The monitoring techniques involving discharge of the batteries have the disadvantage that in the event of any power failure occurring during or within a period of several hours after the monitoring, the batteries may not be able to supply the back-up power since the batteries may not be fully recharged in this interim period. Further, these techniques cannot be employed frequently and therefore the reliability of the power supply always remains uncertain.
It would therefore be desirable to have a simple and no-discharge method to assure the connectivity or presence of the battery bank to the power management systems.